The discovery of over 5000 planets outside the solar system has evermore changed our view of the universe and the tale of other worlds beyond Earth is no longer folklore. In 1989, David Latham found the first planetary candidate HD 114762b (Latham et al., 1989) using the radial velocity technique which was later used to discover 51 Pegasi b as the first exoplanet orbiting a solar-type star by Didier Queloz and Michel Mayor (Mayor and Queloz, 1995). Then back to December 1999, the first transiting exoplanet HD 209458b lightcurve was measured by David Charbonneau (Charbonneau et al., 2002). The gold rush of exoplanet discovery and characterization today is probably beyond the wildest dreams of the early pioneers.From the early days of discovering hot Jupiters, we are now able to study their atmospheres in detail. This thesis focuses on the study of hot Jupiter atmospheres. Hot Jupiters are rare outcomes of planet formation and their origin remains a mystery (Dawson and Johnson, 2018). The study of their atmospheres can help us to understand their formation and evolution history and also develop techniques for future remote sensing of potentially habitable transiting exoplanets in the search for life beyond earth. Just like our solar system with every planet being special and different, the same is true for hot Jupiters. I will first go into detailed studies of 4 individual hot Jupiters (WASP-76b, WASP-74b, HAT-P-41b, and KELT-20b) and then connect them to more broad population-level statistical studies. These four hot Jupiters all have dayside temperatures exceeding 2000K with an inflated radius and a very short (<10 days) orbital period. However, they exhibit different atmospheric features and properties. The relatively cooler ones like WASP-74b and HAT-P-41b show mostly blackbody-like dayside emission spectra which indicate isothermal temperature-pressure profiles. The largely featureless transmission spectrum from WASP-74b is likely caused by clouds condensing in the terminator region. On the other hand, the hotter WASP-76b dayside shows CO emission features with evidence for water thermal dissociation. Combined with heavy metal absorption features seen in the NUV part of the transmission spectrum, gaseous metals are likely causing thermal inversion in the upper atmosphere. KELT-20b shows the strongest water and CO emission features on the dayside despite a similar dayside temperature compared to WASP-76b. The unique A-type host star of KELT-20b is likely the difference-maker. The intense short-wavelength UV/NUV radiation from the A star gets preferentially absorbed in the upper atmospheres by the gaseous metals which heats up the corresponding layers and drives stronger thermal inversion. The dayside emission spectra of these four planets are then compared to all other hot Jupiters with temperatures ranging from ∼1500K to over 4000K. These four planets sit in a key transitional parameter space where we see dayside emission spectra from cooler planets mostly have water absorption features and hotter planets are mostly blackbody-like. This trend shows the cooler (<2000K) planets do not have high altitude absorbers needed for thermal inversion, and the much hotter (>3000K) ones are affected by thermal dissociation of water and rising continuum opacity of H−. Only in this in-between temperature range combined with strong

inversion from gaseous metal absorbers and strong UV radiation, we could see prominent water emission features.I also did a population-level statistical study of all observed transmission spectra focused on the 1.4 μm water band. Each spectrum was fitted to determine the water feature strength which is then normalized by the planetary atmospheric scale height. I found a statistically significant trend of stronger water absorption features as a function of planet equilibrium temperature. This trend can be explained by the presence of aerosols which condense more easily under cooler conditions. Although this study was conducted back in 2017 before my other publications, the trend still holds and provides a valuable statistical comparison study framework for exoplanet atmospheres.